In one class of fuel injector, a plunger is driven downward within the injector to pressurize fuel for each injection event. Between injection events, the plunger retracts and the fuel injector returns to a relatively low pressure state. The plunger can be driven to move in any of a number of ways including via a rotating cam or possibly even via hydraulic fluid pressure from a common rail. In many instances, the plunger and the nozzle portion of the fuel injector are housed in a common injector body. In other instance, these two functions are separated with each nozzle assembly having a dedicated unit pump. In any event, those skilled in the art have generally come to recognize that higher injection pressure levels can generally be exploited to reduce undesirable engine emissions, including but not limited to NOx. unburned hydrocarbons and soot. Most of these fuel injectors include a nozzle valve member that moves between positions that either open or close the nozzle outlets to facilitate spray of fuel into an engine cylinder. These nozzle valve members are usually biased toward their closed position by a compressed biasing spring, but other biasing strategies are available such as by using hydraulic fluid pressure to bias the nozzle valve member toward its closed position. By carefully choosing a pre-load on the biasing spring and adjusting surface areas on the nozzle valve member, along with the possible use of a shim, a valve opening pressure for the nozzle valve member can be reliably and consistently set among a group of fuel injectors. However, there are often urges to increase the valve opening pressure of the nozzle valve member, but doing so by increasing the pre-load on the biasing spring can be problematic. Simply increasing the pre-load on the biasing spring can undermine the fuel injector's ability to inject relatively small amounts of fuel, especially when the engine is operating at a low speed and load condition.
Since the nozzle valve member biasing springs of the prior art inherently bias the nozzle valve member to the same magnitude across the engine operating range, there is also inherently some compromise in choosing a valve opening pressure for the nozzle valve member via a biasing spring pre-load. Those skilled in the art recognize that optimizing the biasing spring pre-load for low speed and low load operating conditions can be entirely different than optimizing the spring pre-load for high speed and load operating conditions. On one hand, excessively low injection pressures can lead to increased production of undesirable engine emissions, while on the other hand, elevated injection pressures can introduce variability issues among fuel injectors via a difficulty in making injectors behave consistently. Thus, there is often a conflict between maintaining acceptable controllability and minimizing variability among fuel injectors verses a motivation to increase mean injection pressure levels and/or the nozzle valve member's valve opening pressure.
The present invention is directed to one or more of the problems set forth above.